As so often with Manchester United this season, it was not pretty. In fact, it was painful to watch, but they kept alive their chance of rescuing their Premier League season and returning to the Champions League.

Louis van Gaal wore shades in the dugout, but his future looked anything but bright while his team were struggling to put pressure on Manchester City in fourth position. It took a catastrophic 72nd-minute mistake from Norwich defender Sebastien Bassong to hand United victory, their 10th 1-0 win of their season -- eight of them in the league.

The Cameroon international misjudged a hopeful, lofted ball from Antonio Valencia to let Wayne Rooney escape behind his back. Rooney, only playing at centre-forward as an emergency measure, showed coolness and strength in slotting in Mata to score a goal that keeps United on the shoulders of City, who must now beat Arsenal on Sunday to hold off their neighbours, who are a point behind them with a game in hand. Norwich, for their part, stayed 19th in the table, and relegation surely looms for them.

Before the match, Van Gaal had depicted this as do-or-die, and yet his team played with little urgency, though were not helped by the loss of Anthony Martial in the warm-up and Matteo Darmian with what appeared to be a serious injury early in the first half.

United's best chance of the first 45 came when Chris Smalling, up for a corner, headed over. Beyond that and Cameron Jerome's header, saved by the legs of David De Gea, little was created in the way of clear-cut chances by either team. Norwich's strategy was to sit back and wait for the counter.

The second period was a tale of Norwich's defence vs. United's attack, with the latter a malfunctioning, uncreative force. In the absence of Martial and the rested Marcus Rashford, Rooney was struggling, though the midfield behind him badly lacked zest.

What pressure United placed on Norwich in the second half came only in territorial terms. Until Mata scored, the visitors did not have a shot on target after Jesse Lingard's 30th-minute drive into the arms of Norwich goalkeeper John Ruddy.

And yet despite their lack of quality, beating West Ham on Tuesday and Bournemouth next Sunday could secure fourth place. It will have to be achieved without Darmian, in for Daley Blind in a defensive reshuffle. The Italian left the field on a stretcher with his left leg in protective strapping after a clash with Russell Martin. The FA Cup final against Crystal Palace on May 21 looks highly unlikely, and his Euro 2016 is in jeopardy.

Juan Mata hit a second-half winner to keep Manchester United in the chase for fourth.

2. Van Gaal's Rashford let-off

Rooney's recent dalliance with being a deep-lying midfielder had to be abandoned. Carrow Road saw him back at centre-forward, though more by accident than design.

Teenager Rashford's rest came after playing 15 straight matches while the far more experienced Blind had played 39 consecutively and was also putting his feet up back in Manchester.

After the FA Cup semifinal at Wembley, where the Frenchman had scored the last-minute winner to deny Everton, Van Gaal had declared that Martial was a winger, yet named him here as a central attacker. That plan was ripped up when Martial pulled up prematch. Rooney was propelled forward and Ander Herrera drafted into midfield.

Bad luck for Van Gaal, though the days when Sir Alex Ferguson would carefully plan his team selections weeks in advance now seem a long time ago. Such accidents keep on happening. This was the third warm-up injury United had suffered in 2016, following David De Gea and Martial being withdrawn just before the two Europa League legs with Midtjylland. The latter had led to Rashford being given his surprise first start against Midtjylland and announcing himself as United's find of the season, but there would be no such luxury here.

Rooney was unable to supply the speed that a combination of the young Mancunian and Frenchman have lately brought United. Home fans were also to be heard comparing Rooney unfavourably to Leicester speed machine Jamie Vardy. And though Memphis Depay does possess pace, his lack of radar and variety make him galaxies less of a proposition than Martial.

At least Rooney and Mata, two players of poise and vision when granted time and space, were able to make the best of the golden chance that Bassong granted them. Otherwise, Van Gaal would have paid a heavy price for his reshuffling.

Wayne Rooney was back up front as Marcus Rashford was rested for the trip to Carrow Road.

3. Norwich set for relegation

Norwich had more than a fighting chance of staying up, but not so much now. An improved performance against Arsenal last week had brought only a 1-0 loss, yet a path to safety could be plotted by a result here and two winnable closing matches, Watford at home on Wednesday and at Everton next Sunday.

They began like the proverbial train. De Gea's knees deflected away that goalbound Jerome header during an early onslaught on a disorganised United defence rocking back on its heels. But then came circumspection and the train coming to a halt, as Norwich were lulled into the quagmire the game swiftly became.

Sitting back was presumably part of manager Alex Neil's strategy, though rationing attacks when the team had not scored a Premier League goal since April 2 seemed somewhat risky. That, paired with a noticeable lack of urgency when the ball went dead, suggested that gaining a point here and hoping for better in those last two games and bad fate to befall Sunderland and Newcastle was part of the plan.

Half-chances came and went the way of Gary O'Neil and Jerome as this began to look a game where all three points might be snaffled.

Then, disastrously, came Bassong's mistake, and though Norwich still live in hope, they must find goals from somewhere and fortune elsewhere to save their skins.

John Brewin is a staff writer for ESPN FC. Follow him on Twitter @JohnBrewinESPN.